Shape Memory Alloy Wire With Controlled Energy Damping

ABSTRACT

There is provided a shape memory alloy wire with a length of polycrystalline shape memory alloy having an alloy composition including at least one member selected from the group consisting of Cu in at least 10 wt. %, Fe in at least 5 wt. %, Au in at least 5 wt. %, Ag in at least 5 wt. %, Al in at least 5 wt. %, In in at least 5 wt. %, Mn in at least 5 wt. %, Zn in at least 5 wt. % and Co in at least 5 wt. %, and having a martensite crystal structure consisting of one of 2H, 18R1, M18R, and 6R. The length of polycrystalline shape memory alloy has a cross sectional wire diameter greater than 1 micron and less than 500 microns, an oligocrystalline morphology including polycrystalline grains that span the wire diameter and a wire surface with a surface roughness that is no greater than about 100 nanometers.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/036,495, which is the National Stage of International Application No.PCT/US2013/70224, filed Nov. 15, 2013, the entirety of which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under ContractW911NF-07-D-0004, awarded by the Army Research Office. The Governmenthas certain rights in the invention.

BACKGROUND

This invention relates generally to functional materials, such as shapememory alloys, and more particularly relates to control of energydissipation in a superelastic shape memory alloy structure.

The degree of energy dissipation, or energy damping, in a functionalmaterial, such as a shape memory alloy (SMA), has important practicalimplications for many SMA devices and systems. For a wide range of SMAapplications a high degree of damping can be desired, e.g., for enablingimpact absorption and vibration control. In contrast, in other SMAapplications, including mechanical actuation and energy harvesting,energy damping can be undesirable. Aside from this consideration forenergy dissipation, the design specifications for a SMA application as awhole or a SMA active element in an application system can otherwise besimilar. For example, a SMA wire element design that is designed foractuating the movement of a mirror is can also operate in a woven fabricthat is designed dissipating energy from the vibrations in an enginemount.

Shape memory alloys are characterized by a solid-to-solid reversiblephase transformation between a higher temperature phase, calledaustenite, and a lower temperature phase, called martensite. The alloycrystal structure of the austenitic phase is typically a cubicsuperlattice, while the alloy crystal structure of the martensitic phaseis monoclinic or orthorhombic. The transformation of the alloy materialbetween these two phases results in recoverable strains on the order of6-10%. During such a so-called martensitic transformation, energy isdissipated as heat; the amount of this energy dissipation is reflectedin the degree of hysteresis in a phase transformation cycle: the largerthe phase transformation cycle hysteresis the more energy is dissipatedby the phase transformation cycle. As a result, the amount of energydamping produced by a shape memory alloy structure in a phasetransformation cycle can be measured by the size of the hysteresis in astress-strain curve for a phase transformation cycle that is obtainedduring an observed mechanical phase transformation of the structure.

For a range of SMA applications a large hysteresis can be desirable,e.g., for applications in which the function of the SMA is to dampvibrational energy. In such applications for which energy dissipation isdesirable, a SMA material element can be correspondingly engineered todamp mechanical energy. But although a SMA damping design can in generalbe effective, it typically requires a trade-off with other SMA materialproperties, such as mechanical fatigue and corrosion resistance.Similarly, a SMA material element can be engineered to producerelatively low mechanical damping, but also at a trade-off with otherSMA material properties, such as temperature sensitivity, mechanicalstresses, cost, or manufacturability. Due to these inherent trade-offsrequired in the design of a SMA material element with a selected degreeof damping, control of SMA material element damping is oftenimpractical, and results in a common SMA material element design beingemployed for both high-damping and low-damping applications; e.g., witha substantially identical SMA element design being employed for both foractuating structures and for energy dissipating structures.

The trade-offs required for achieving a particular, prespecified degreeof energy damping are particularly large for microscale and nanoscaleSMA structures having SMA material element dimensions in the microscaleor nanoscale. For such SMA structures, the material requirements set byoperational and performance considerations can be very stringent. Inparticular, the compromises that are often required for achievingvarious small-scale operational performance can result in an inabilityto selectively control energy damping by the SMA structure. As a result,microscale and nanoscale SMA material structures can be severely limitedin meeting specific energy damping requirements given for microscale andnanoscale SMA applications.

SUMMARY

There is provided herein a shape memory alloy wire of a length of apolycrystalline shape memory alloy having an alloy composition includingat least one member selected from the group consisting of Cu in at leastabout 10 wt. %, Fe in at least about 5 wt. %, Au in at least about 5 wt.%, Ag in at least about 5 wt. %, Al in at least about 5 wt. %, In in atleast about 5 wt. %, Mn in at least about 5 wt. %, Zn in at least about5 wt. % and Co in at least about 5 wt. %. The shape memory alloycomposition has a martensite crystal structure consisting of one of 2H,18R₁, M18R, and 6R. The length of polycrystalline shape memory alloy hasa cross sectional wire diameter greater than about 1 micron and lessthan about 500 microns, and has an oligocrystalline morphology includingpolycrystalline shape memory alloy grains that span the cross sectionalwire diameter. The length of a polycrystalline shape memory alloy has awire surface with a surface roughness that is no greater than about 100nanometers.

The controlled energy damping enabled by this shape memory alloy wirecan be used in a variety of applications to enable optimal performanceof smart materials in actuation, mechanical vibration control, energyharvesting, and other applications. Other features and advantages of theenergy damping control method will be apparent from the followingdescription and accompanying figures, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plot of stress hysteresis, Δσ, or amount of energydissipated for a SMA martensitic transformation cycle that is measuredfor martensitic transformation of a SMA wire as a function of wirediameter, identifying a size regime in which volume effects dominate(I), a size regime in which surface effects dominate (II), and a sizeregime in which surface and volume effects are starved and sizedominates (III);

FIG. 1B shows two plotted example relationships between energy dampingand surface roughness, shown as stress hysteresis, Δσ, as a function ofSMA structure surface roughness;

FIG. 2 is a schematic side view of a SMA structure having a surface withtwo different scales of surface roughness;

FIGS. 3A-3E are schematic views of an example superelastic alloymicro-pillar, wire or fiber, planar structure, open-cell foam shape, andtube, respectively, having surfaces that can be modified as providedherein;

FIGS. 4A-4E are schematic views of an example superelastic alloycantilever, membrane, bridge, ribbon, and vertical wall, respectively,having surfaces that can be modified as provided herein;

FIGS. 5A-5C are schematic views of an example weave of superelasticalloy fibers, bundle of superelastic alloy fibers, and braid ofsuperelastic alloy fibers, respectively, having surfaces that can bemodified as provided herein;

FIG. 6 is a schematic view of a spring actuator including an SMA springelement having a surface smoothness for long actuation fatigue life;

FIG. 7 is a schematic view of a energy damping structure including SMAwires have surface roughness for damping energy;

FIGS. 8A-8B are atomic force microscopy scans showing the surfacetopography of a SMA wire with a large surface roughness and a smoothsurface finish obtained after surface polishing; and

FIGS. 9A-9B are plots of experimentally-measured stress-strain curvesfor a SMA wire having a rough surface and an SMA wire having a smoothsurface, for a first martensitic transformation cycle and a tenthmartensitic transformation cycle, respectively.

DETAILED DESCRIPTION

The energy dissipation, or energy damping, characteristics of asuperelastic structure, such as a shape memory alloy (SMA) structure,are influenced by surface and volume effects of the SMA structure, withthree distinct size regimes of influence being distinguishable. Hereinis provided a methodology for the particular tuning of SMA energydamping by control of surface effects in a corresponding one of thedistinct size regimes. Referring to FIG. 1A, this size influence isillustrated in an example plot of stress hysteresis, Δσ, or amount ofenergy dissipated for a cycle of SMA austenite-martensite phasetransformation, as a function of the size of a SMA structure feature,here the diameter of SMA wires. In this example, measured data fromCu-based SMA wires is presented, specifically for Cu—Zn—Al and Cu-al-Nisystems, but the general relationship is not limited to Cu-based SMAwires and is applicable in general to SMA structures.

In a first size regime (I), bulk effects of the SMA material, i.e., inthe material volume, such as volume defects, influence the stresshysteresis of the SMA structure and result in relatively low stresshysteresis and low energy damping for a SMA structure extent above theregime I lower size threshold. In a second size regime (II), surfaceeffects of the SMA structure, such as surface roughness, impact themechanical response of the structure, and in this regime, as thestructure feature size is reduced, stress hysteresis and energy dampingincreases. In a third regime (III), below a characteristic feature sizethreshold, the scale of the SMA structure dominates, and starvation ofsurface defects occurs. This third regime is characterized by arelatively high stress hysteresis and high energy damping.

In the experimental data of the plot of FIG. 1A, relating to Cu-basedSMA alloy wires, the first, volume-dominated size regime (I), occurs atSMA feature sizes greater than about 500 μm. The second,surface-dominated size regime (II) occurs at SMA feature sizes betweenabout 1 μm and about 500 μm. The third, size-dominated regime (III)occurs at SMA feature sizes less than about 1 μm. These particularregime thresholds are specific to this Cu-based SMA alloy wire exampleonly, but demonstrate the three size-based regimes that are applicableto SMA structures in general.

It is discovered herein that for the surface-dominated feature sizeregime (II) of a SMA structure, control of the surface finish of thestructure can be particularly specified to produce a correspondingenergy damping characteristic for the structure. During a martensitictransformation cycle in a SMA structure, frictional energy is dissipatedas heat when the austenite/martensite interface moves past obstacleswithin the SMA structure and at the structure surface. The verticalextent of the hysteresis loop that is characteristic of thetransformation cycle, i.e., the height of the loop formed in a plot ofstress as a function of applied strain of an austenite-martensitesuperelastic transformation cycle is proportional to the amount ofenergy dissipated in the cycle, which is the energy that is damped inthe cycle. In an SMA structure having at least one structural featurewith a length, or extent, that falls within the surface-dominated regime(II) of size for that particular SMA structure, obstacles at the surfaceof the structure can be a dominant source of energy damping during thetransformation cycle. For such structures in this surface-dominatedregime, the amount of energy dissipated in one superelastic cycle isintimately linked to the surface finish of the structure.

By controlling the surface roughness of such structures, both the sizeof the population of surface obstacles as well as the frictionalpotential of the surface obstacles can be tuned. For example, bysmoothing a SMA structure surface, the number of obstacles at thesurface can be reduced, such that fewer surface obstacles exist forencounter with the austenite/martensite interface movement during aphase transformation, with a corresponding reduction in energydissipation during the transformation. Alternatively, by roughening aSMA structure surface, the number of obstacles at the surface can beincreased, such that more surface obstacles exist for encounter with theaustenite/martensite interface movement during a phase transformation,with a corresponding increase in energy dissipation during thetransformation. Such surface tuning is employed to control the desiredamount of energy to be dissipated by the structure during a superelastictransformation cycle. As explained in detail below, this method ofcontrolling damping can be used in a variety of applications to enableoptimal performance of smart materials in actuation, mechanicalvibration control, energy harvesting, and other applications.

For SMA structures in the volume-dominated size regime (I), having alarger feature size than that in the surface-dominated size regime (II),the surface-to-volume ratio is too small for the surface finish of thestructure to play any important role in the energy dissipation during asuperelastic transformation cycle. On the other hand, for very small SMAstructures, in the third size regime (III), surface obstacles are ingeneral too few and far between to be the dominant source of energydissipation; instead the mechanical behavior of the SMA structure duringa superelastic transformation is here controlled by defect probability.As a result, damping performance is dominated by surface condition onlyfor SMA structures having a feature with a length scale in thesurface-dominated energy dissipation size regime (II) that ischaracteristic of that structure. For the example Cu-based SMA structurerepresented by the data plot in FIG. 1A, that characteristic regime isbetween about 1 μm and about 500 μm.

This surface-dominated energy dissipation regime (II) can be determinedfor any SMA structure to ascertain the size range of structural featuresfor which the state of the structure surface can be tuned to control theenergy damping of the structure during a superelastic transformationcycle. To make this determination of the surface-dominated energydissipation regime for a selected SMA structure, there can be employedany suitable analysis or technique, including empirical and experimentaltechniques.

In general, a surface-dominated size regime can be characterized, thatis, can be well-defined, for a SMA structure having a surface area thatis large relative to the structure's volume and/or for a SMA structurethat includes structural defects having an average, or characteristicdefect size that is small relative to the volume of the SMA structure.Structural defects can include, e.g., dislocations, dislocation tangles,vacancies, inclusions, second phases, and other such features, and canvary from SMA material to SMA material. For many materials, such defectshave characteristic sizes or size ranges. For SMA structures includingdefects the size of which are small relative to the SMA structurevolume, a surface-dominated size regime can in general be determined forthe SMA structure. Measurement techniques that enable the determinationof such defect size relative to structure volume can therefore beconducted to confirm a surface-dominated size regime for a given SMAstructure. Analysis of a structure surface area relative to structurevolume can also be conducted to confirm a surface-dominated size regimefor a given SMA structure.

Additional analysis and techniques can be employed to confirm asurface-dominated size regime for a given SMA structure. For example, ifa correlation between surface-to-volume ratio of the SMA structure andsome mechanical property, like damping, is observed, then the structuredoes have a surface-dominated size regime and the approximate range ofthat regime can be determined. Experimentally, a given SMA structure canbe cut in half and the surface of one half either polished or roughenedrelative to the surface of the other half, so that the two halvespossess similar properties but different surface roughnesses. Themechanical response of superelastic martensitic cycling of the twosamples reveals if the structure is in a surface-dominated size regime:If the hysteresis of the martensitic cycling of the two samples differsgreatly between the two samples, then the structure size is in thesurface-dominated regime, while if the hysteresis is similar for the twosamples is similar then the structure size is not in the surfacedominated regime.

In one example of experimental testing, multiple samples of a selectedSMA structure design, such as a SMA wire, are produced with a range offeature sizes that could be within the surface-dominated size regime,e.g., a range of SMA wire diameters. The SMA samples preferably all havecommon characteristics other than feature size, and preferably havesubstantially identical surface condition, including surface roughness.The selected feature size is then adjusted for each of the SMA samples.In SMA materials including Cu, the surface-dominated size regime hasbeen established, as shown in FIG. 1, to be between about 10 microns andabout 200 microns, and such can be confirmed for a given Cu-based SMAstructure by producing samples having feature sizes within and aroundthis range. For other alloy systems, feature sizes around this range ofbetween about 10 microns and about 200 microns can also be produced forinitial analysis. For some SMA structure sizes, review of publishedliterature can be preferred to initially confirm that asurface-dominated size regime exists for the SMA material and structure,and to ascertain general expectations for the surface-dominated sizeregime size boundaries.

After initial analysis, a specific range of feature sizes can beexperimentally tested to determine the boundaries of thesurface-dominated size regime. The size interval between sizes ofdifferent samples produced for experiment can be set based on theaccuracy required for a given application. For example, feature sizeintervals of 10 microns can be employed as a starting point. The numberof different feature size samples to be produced is therefore also setbased on the accuracy required for a given application. At least aboutfive samples, e.g., 10 or more samples, can be employed as-needed for agiven application. The produced samples are then each exposed to a knownstrain that causes martensitic transformation, and one or moretransformation cycles are imposed, with the energy damping of each cyclemeasured. Each of the three size regimes like those shown in the plot ofFIG. 1A can then be identified. The surface-dominated size regime isthat range of feature sizes for which the transformation cycle energydamping shows a dependence of the degree of energy damping on thefeature size. The other size regimes I and III are not characterized bya dependence of transformation cycle energy damping on feature size; asshown in the example plot of FIG. 1A, in regimes I and III the energydamping is relatively constant for any feature size. The feature sizeboundaries between regimes I and II and between regimes II and III cantherefore be determined by testing samples having feature sizes thatproduce conditions of energy damping dependence on feature size andenergy damping independence of feature size.

Once a given SMA structure is characterized to identify the three sizeregimes for that structure, the surface-dominated size regime can befurther characterized. In particular, it can be preferred for a selectedSMA structure to determine the particular functional relationship thatexists for that SMA structure between the degree of surface roughness ofthe structure and the martensitic transformational energy dampingproduced for that surface roughness. In one experimental technique todetermine such, multiple samples of the selected SMA structure can befabricated including a common structural feature having a size extentthat is within the surface-dominated size regime identified for the SMAstructure. Each sample is processed to possess a different, knownsurface roughness while the other physical properties of the structureare maintained equal. For example, the surface roughness of each in aset of multiple SMA wires can be modified to result in a surfaceroughness that is between about 1 nm and about 500 nm, with sampleshaving differing degrees of roughness at roughness intervals of, e.g.,50 nm. Mechanical testing of the wires with different surface roughnesscan then be conducted by, e.g., applying a known strain to each sampleand measuring the stress hysteresis of the sample during one or moresuperelastic transformation cycles. Measurement of the stresshysteresis, corresponding to energy damping, of each of the samples,establishes the relationship between hysteresis size and surfaceroughness for the surface-dominated size regime of the structure.

FIG. 1B includes two example plotted relationships between energydamping and surface roughness, shown as stress hysteresis, Δσ, or amountof energy dissipated for a transformation cycle, as a function ofsurface roughness, R_(q). The relationships shown here are onlyschematic in nature; in general, a given SMA material, structuralarrangement, and surface condition results in a distinct, different formof the relationship. But whatever particular relationship form existsfor a given SMA structure and surface roughness, it is understood thatthe relationship is monotonically increasing, i.e., a larger surfaceroughness leads to more or the same damping as that produced by smallersurface roughness. A reduction in the surface roughness of a SMAstructural feature in the surface-dominated size regime results in areduction in energy damping by the structure during a martensitictransformation cycle, as shown in the plots. Again, it is recognizedthat the relationship between energy damping and surface roughness canbe different for distinct SMA materials and structural arrangements, andno one relationship can be applicable in general. It can therefore bepreferred for a given SMA material and/or structural featureconfiguration to characterize the material and feature to determine theparticular correspondence between energy damping and surface roughnessfor that SMA material and feature.

With such a relationship established, there can a priori be prespecifieda desired degree of energy damping for the SMA structure for an intendedapplication and the surface of the structure suitably processed with asurface roughness that produces the prespecified damping. Relationshipslike those plotted in FIG. 1B can be implemented in hardware or softwareto enable automatic determination of a required surface roughness forachieving a desired degree of energy dissipation. For example, given aSMA structure having a structural feature in the surface-dominatedenergy damping regime, the structure can be exposed to selected processconditions that modify the surface roughness to an amount of roughnessthat is specified by the determined relationship to provide apreselected energy damping. For example, the surface roughness of thestructure can be decreased, for example by polishing the SMA structure,to achieve some relatively low degree of energy damping. Such acondition is desirable, for example, for SMA actuator applications inwhich small energy losses and long fatigue life are required. Thesurface roughness alternatively can be increased by an appropriatesurface modification technique to achieve some specified relativelylarge degree of energy damping. Such a condition is desirable, forexample, for SMA devices purposed to damp energy of, e.g., mechanicalvibrations, impact, or sound. Thus, once a direct relationship betweensurface roughness and surface-dominated energy damping is fullycharacterized for a given SMA material structure, the SMA materialstructure can be engineered to produce a prespecified degree of energydamping. The SMA material structure can then be employed in a wide rangeof applications with differing energy damping requirements bycorrespondingly controlling the surface roughness based on the energydamping characteristic.

In control of a SMA structure surface to produce a prespecified degreeof energy damping the structure surface can be processed to take on auniform condition across the structure surface, or can include acustomized surface topology having multiple layers of surface roughnessfeatures that are each on a different size scale. Such topologycustomization can be employed, e.g., to enable control of both energydamping and one or more other mechanical properties. For example, for anSMA application requiring a high degree of energy damping as well asextended fatigue life, the SMA structure's surface roughness can betuned to meet both goals by introducing two levels of roughness, onemicroscopic, or local, and one macroscopic, or global. FIG. 2schematically illustrates an example of such a modulated surface 10 of asuperelastic SMA structure 12. On a relatively small size scale, 14, thestructure 12 exhibits a first surface roughness extent, while on arelatively larger scale, 16 the structure exhibits a second surfaceroughness extent.

In the example provided in FIG. 2 the local-scale surface roughness,which in general impacts SMA actuator fatigue life, is low: the surfaceis locally smooth. The surface roughness at the larger scale 16, isconversely relatively rougher, to enable a selected, relatively highdegree of energy damping. With this combination of different degrees ofsurface roughness at differing size scales, the SMA structure cansimultaneously possess operational characteristics that areconventionally in opposition. Thus, a range of SMA structurecharacteristics, including damping, can be addressed by controlling theshape and character of the structure's surface by appropriate surfacemodification technique.

This control of surface roughness or smoothness is generally applicableto any SMA structure provided that the length of one or more structuralfeatures of the structure is within the size regime in which themechanical properties of the structure are surface-dominated in themanner defined above. Any SMA morphology can be employed, includingsingle crystalline SMA structures, polycrystalline SMA structures, andother SMA morphologies. For example, oligocrystalline SMA structures,defined herein as having a larger surface area than internal grainboundary area, can also be employed, e.g., with a bamboo structure, asdescribed below.

For these various morphologies, any in a wide range of SMA systemsshowing superelasticity and/or shape memory properties can be employedwith the surface control methodology herein. In general, the selectedSMA material preferably is characterized by a phase transformationbetween austenite and martensite that is reversible. In addition, formany applications, it can be preferred to distinguish betweennickel-titanium alloys, also known as Nitinol or Ni—Ti, and alloys thatare not based on nickel and titanium, such as Cu—Al—Ni, Cu—Zn—Al,Cu—Al—Be, Fe—Mn—Si, Ni—Mn—Ga and others. In Ni—Ti alloys the martensiticphase transformation is between the high temperature austenite phasehaving a B2 crystal structure and belonging to the Pm3m space group andthe low temperature martensite phase having a B19 crystal structure andbelonging to the P₂₁/m space group. While other shape memory alloysystems also transform from austenite to martensite, the martensitephase, and often the austenite phase as well, belong to a crystalstructure and space group that are different than that of Ni—Ti.

For many applications, it can be preferred to employ the surface controlmethodology herein with an alloy system having a transformation otherthan the Ni—Ti B2 austenite to B19′ martensite transformation. Alloysystems characterized by other transformations, including those having aB2 austenite structure and a martensite structure that is not B19′ withspace group P₂₁/m can be preferred. The austenite phase in alloy systemsother than Ni—Ti is often cubic, such as L₂₁, D_(O3) or B2 or others,with space groups Fm3m, Pm3m or others. Table I below provides a list ofexample martensite crystal structures, in Ramsdell notation, and examplespace groups of martensite crystal structures of alloys that can bepreferred for the surface conditioning methodology herein.

TABLE I Martensite Crystal Structure Martensite Space Group 2H Pnmm I8R₁A2/m M18R Im3m 6R A2/m

Table II below provides a listing of many example SMA materials that arewell-suited for the surface conditioning methodology and that do nottransform from austenite to the B19′ martensite crystal structurebelonging to the P2₁/m space group. In addition, it can be preferred,for many applications, to employ an SMA material that is characterizedby a composition that includes at least about 10 wt. % Cu oralternatively, an SMA material composition that includes at least about5 wt. % of one or more of the elements Fe, Au, Ag, Al, In, Mn, and Co.In other words, for some applications, it can be preferred that the SMAcomposition include at least one of Cu in at least about 10 wt. %, Fe inat least about 5 wt. %, Au in at least about 5 wt. %, Ag in at leastabout 5 wt. %, Al in at least about 5 wt. %, In in at least about 5 wt.%, Mn in at least about 5 wt. %, Zn in at least about 5 wt. % and Co inat least about 5 wt. %.

TABLE II ALLOY COMPOSITION (atomic %) Ag—Cd 44-49 Cd Au—Cd 46.5-48.0 CdAu—Cd 49-50 Cd Cu—Zn 38.5-41.5 Zn Cu—Sn 14-16 Sn Cu—Zn—X, with X = Si,Sn, Al, Ga A few at % Cu—Al—Ni 28-29 Al, 3.0-4.5 Ni Cu—Al—Mn 16-18 Al,9-13 Mn Cu—Au—Zn 23-28 Au, 45-47 Zn Cu—Al—Be 22-25 Al, 0.5-8 Be In—Tl18-23 Tl In—Cd 4-5 Cd Mn—Cd 5-35-Cd Fe—Pt 25 Pt Fe—Ni—Co—Ti 23 Ni, 10Co, 10 Ti Fe—Ni—Co—Ti 33 Ni, 10 Co, 4 Ti Fe—Ni—Co—Ti 31 Ni, 10 Co, 3 TiFe—Ni—C 31 Ni, .4 C Fe—Ni—Nb 31 Ni, 7 Nb Fe—Mn—Si 30 Mn, 28-33 Mn, 4-6Si Fe—Cr—Ni—Mn—Si 9 Cr, 5 Ni, 14 Mn, 6 Si Fe—Cr—Ni—Mn—Si 13 Cr, 6 Ni, 8Mn, 6 Si Fe—Cr—Ni—Mn—Si 8 Cr, 5 Ni, 20 Mn, 5 Si Fe—Cr—Ni—Mn—Si 12 Cr, 5Ni, 16 Mn, 5 Si Fe—Mn—Si—C 17 Mn, 6 Si, 0.3 C Fe—Pd 30 Pd

Whatever SMA alloy composition is selected to be employed with thesurface conditioning methodology, the material is provided with at leastone structural feature having an extent, or size, that is within thelength scale determined for the SMA material to be characterized as thesurface-dominated size regime. This size regime can be imposed on thestructure in any suitable fashion for a desired SMA application, and noparticular structural feature arrangement or configuration is required.An SMA structure having controlled surface finish can take any suitableform, and is not limited to the example wire structure described above.The SMA structures are not limited to a particular morphology, and canexhibit an oligocrystalline, polycrystalline, or monocrystallinemicrostructure. For any morphology, and referring to FIGS. 3A-3E, FIGS.4A-4E, and FIGS. 5A-5C, the SMA material can be employed in thefabrication of a structure having a feature such as a wire or wire-likerod, as in FIGS. 3A-3B, having a diameter, d, within thesurface-dominated size regime. Any morphology can be employed, but ifthe material is polycrystalline, there can be imposed an additionalconstraint requiring that the diameter, d, be no larger than the extentof a polycrystalline grain 20 of the structure. As shown in FIG. 3B,this results in the oligocrystalline bamboo structure in which grainsgenerally spanning the diameter of the wire are configured along thelength of the wire.

Turning to FIG. 3C, there is shown an example of a SMA structureprovided in the configuration of a superelastic alloy film, a layer, ora planar structure 28. The planar structure 28 is characterized by athickness, t, that is produced to be within the size regime forsurface-dominated energy dissipation by the planar structure. Thisthickness of the planar structure can be further controlled, if desired,to be no larger than the extent of a grain 20 of the structure, wherebygrains generally span the entire thickness of the structure.

In FIG. 3D there is shown a further example of suitable SMA structure,here in the configuration of superelastic alloy open cell foam 30 havingstruts throughout the foam; similarly a closed cell foam can beemployed. The span, w, of an individual cell strut is produced to bewithin the size regime for surface-dominated energy dissipation by theSMA structure. The span further can be specified to be no larger thanthe extent of a grain 20 of the structure, whereby grains generallyextent across the entire strut span of the foam. In FIG. 3E, there isshown a further example of a suitable SMA structure, here in theconfiguration of a superelastic alloy tube 29 having a tube wallthickness, x. The wall thickness, x, of the tube is produced to bewithin the size regime for surface-dominated energy dissipation by theSMA structure. The wall thickness further can be produced to be nolarger than the extent of a grain 20 of the structure, whereby grainsgenerally span the entire thickness of the tube wall, if desired for aselected application.

SMA structures can be configured with a wide range of superelastic alloystructural elements in any suitable manner to produce a desiredstructure arrangement for a given application and with surfaceconditioning of the structure. For example, referring to FIG. 4A, aplanar SMA alloy structure can be configured as a cantilever beam 32supported on a substrate 34. As shown in FIG. 4B, a planar alloystructure can be configured as a free standing, self-supported plate ormembrane 36 supported at the membrane edges by a substrate 34.Alternatively, an arching bridge-like alloy surface structure 38 can beprovided on a support or substrate 34. Other configurations, like thatin FIG. 4D, such as an alloy ribbon 40 that is free to be disposed orincorporated into a structure, can be produced in a SMA structure.Referring to FIG. 4E, a planar alloy structure 42 can also be arrangedvertically relative to a substrate 34 or other structure. In each of theexample structures of FIG. 4, one feature, such as beam, membrane,bridge, or ribbon thickness, is within the size regime specified forsurface-dominated energy damping by the structure.

In general, SMA structural elements and geometrical features that arenot within the surface-dominated energy damping size regime can also beincluded in the SMA structure, and features that are not superelasticcan also be included and incorporated into the structure. Suchnon-superelastic elements can be in contact with or connected to thesuperelastic alloy in any suitable configuration that enables phasetransformation of the superelastic alloy. In addition, two or moredistinct compositions of superelastic alloy can be included in thestructural configuration.

SMA fibers or wires can similarly be configured in any suitablearrangement for surface conditioning to achieve a selected energydamping. Referring to FIG. 5A, superelastic alloy wire or fiber 25 canbe woven into a fiber sheet 45 to form a SMA structure that is a wovensheet of layer that can be employed, e.g., as a fiber textile in themanner of fabric. Such alloy wires or fibers can be bundled, as shown inFIG. 5B, in a cable or bundle 48 of fibers 25 that are twisted, braided,intertwined, or otherwise configured within the bundle for a selectedapplication, including coaxial arrangements. As shown in FIG. 5C, fibersor wires 25 can be braided in a braiding configuration 50 for producinga braided sheet, tube or other configuration of wires or fibers. Theweave or braid in such a configuration can be two-dimensional, or can becharacterized by any suitable multi-dimensional weave or braid in thedirectionality of the build-up of the structure. In such structurescomprising more than one individual wire or fiber, one or more of thefibers or wires can be superelastic alloys, with one or morenon-superelastic fibers or wires included in the braid or weave. In sucha composite arrangement, such as a felts, where SMA wires can be spreadin the weave or braid matrix to provide isotropic energy dissipationthrough the structure. Alternatively, all of the fibers or wires in thestructure can be of one or more superelastic alloy compositions.

In the example structures of FIGS. 3, 4, and 5, at least one feature ofthe structures is characterized by an extent that is within the sizeregime for surface-dominated energy dissipation during martensitictransformation of the structure. For many applications, this featuresize extent is less than about 1 mm and more than about 1 μm; that is, afeature, such as wire diameter, foam strut diameter, film thickness,ribbon thickness, beam or bridge cross-sectional thickness, tube wallthickness, or other feature extent is no greater than about 1 mm and noless than about 1 μm produces feature behavior in the surface-dominatedsize regime of energy dissipation during a martensitic transformationcycle. For many applications, a feature size extent of less than about500 μm and more than about 1 μm can be specified to produce featurebehavior in the surface-dominated size regime of energy dissipation.

With these material and dimensional specifications, there is selected aSMA material structure and feature geometry to be customized by surfaceconditioning for producing a selected degree of energy damping. In oneexample of such, there is provided a superelastic wire of an alloy ofCu—Zn—Al having a wire diameter that is between about 20 μm and about200 μm, that renders that structure oligocrystalline and in thesurface-dominated energy dissipation size regime.

With a selected SMA material arranged in a surface-dominated structuralgeometry, the surface roughness of the SMA structure can be modified toachieve a surface condition that produces a selected degree of energydamping. The structure can be exposed to any suitable process conditionsthat achieve a desired surface smoothness, by smoothing or rougheningthe surface; no particular surface processing is required. The followinglist recites examples of roughening and smoothing techniques: blanching;mechanical polishing and/or grinding; chemical, electrochemical ormechanical etching, case hardening; ceramic glazing; cladding; coronatreatment; diffusion processes such as carburizing or nitriding;electroplating; galvanizing; gilding; glazing; knurling; painting;passivation/conversion coating by, e.g., anodizing, bluing, chromateconversion coating, phosphate conversion coating, parkerizing, or plasmaelectrolytic oxidation; plasma spraying; powder coating; thin-filmdeposition of a selected material or materials in a coating or otherlayer on the surface of the SMA structure, e.g., by chemical vapordeposition (CVD), electroplating, electrophoretic deposition (EPD),mechanical plating, sputter deposition, physical vapor deposition (PVD),vacuum plating, or other deposition process; vitreous enameling;abrasive blasting, such as sandblasting; burnishing, polishing, such aschemical-mechanical planarization or polishing (CMP) and buffing;electropolishing; flame polishing; gas cluster ion beam processing;grinding; industrial etching; linishing; mass finishing processes suchas tumble finishing and vibratory finishing; pickling; peening, such asshot peening; superfinishing, such as magnetic field-assisted finishing;and other suitable surface conditioning processes.

Many of the surface conditioning techniques recited in the list abovecan be adapted for either roughening or smoothing a SMA structuresurface as-desired. For example, the conditions of mechanical polishingwith abrasive paper can be adapted for either roughening or smoothing asurface to correspondingly increase or decrease energy damping,respectively. Course abrasive paper, with a relatively low grit number,can be employed in mechanical polishing to increase surface roughness,while fine abrasive paper, with a relatively high grit number, can beemployed in mechanical polishing to reduce surface roughness. Similarly,the conditions of surface electropolishing can be adapted to produceeither a smooth or rough surface finish. The electropolishing voltagecan be controlled to produce conditions that smooth or roughen asurface, and a voltage that is either higher or lower than a particularpolishing voltage for a given material can be employed to producemicroscopically smooth surfaces with large macroscopic undulations. Inthis way, a hierarchical surface roughness, as discussed above and shownin FIG. 2, can be produced.

Other surface conditioning techniques further can be customized forsurface roughening or smoothing. For example, the conditions of chemicaletching can be adapted to either roughen or smooth a surface. While awide range of chemical etchants render a surface smooth, other chemicaletchants roughen a surface, and the SMA structure composition andmorphology can be exploited to tailor the effects of chemical etching,e.g., to remove surface defects or to produce surface erosion. Theseexamples demonstrate that a wide range of materials processingtechniques can be tailored and adapted to provide a selected degree ofSMA surface roughening or surface smoothing in a methodology to controlenergy damping in an SMA structure having a feature size in thesurface-dominated size regime.

With a selected surface conditioning process, the SMA structure surfacecan be characterized by a degree of surface roughness, e.g., a root meansquare surface roughness, R_(q), and/or an arithmetic average surfaceroughness, R_(a). Whatever roughness metric is employed, as explainedabove the surface roughness can be correlated directly to acorresponding SMA structure energy dissipation during a martensitictransformation cycle. For applications in which low SMA martensitictransformation energy dissipation is called for, a SMA structure surfaceroughness, R_(q), that is no greater than about 100 nm, and anarithmetic average surface roughness, R_(a), that is no greater thanabout 80 nm, can be preferred. For applications in which a larger energydamping is desired, the surface roughness, R_(q), can be tuned to alarger value, for example R_(q) in the range of about 100 nm-150 nm,about 150 nm-200 nm, about 200 nm-250 nm, about 250 nm-300 nm, about 300nm-400 nm, about 400 nm-500 nm, about 500 nm-1000 nm, or larger.Conversely, for applications requiring a more smooth SMA structuresurface to achieve a lower energy damping, an average surface roughness,R_(a), in the range of about 10 nm-50 nm, about 50 nm-80 nm, about 80nm-100 nm, about 100 nm-200 nm, about 200 nm-500 nm, about 500 nm-1000nm can be deemed desirable. As the surface roughness is increased, theamount of damping that is achieved during a martensitic transformationcycle increases, and can be correspondingly tuned to the degree that isrequired for a particular application.

The surface conditioning methodology thereby can be implemented in thedesign and manufacturing of a wide variety of applications, includingsensing and actuation, superelastic movement, and energy damping, e.g.,for medical devices, smart fabrics, and sensing. The following listprovides a number of examples in which an SMA structure having a featurein the surface-dominated size regime can be engineered by the surfaceconditioning methodology to attain a requisite degree of energydissipation, and is not limiting: medical devices, such as insulinpumps, drug release devices, guides for catheters through blood vessels,steerable medical instruments like guide wires and guide pins, bonesuturing anchors, suture retrievers, remote suturing or stapling andsteering devices, stents, pulmonary embolism filters, and gall stonecollectors, orthotics, orthodontic bridge wires and endodontic files;actuators, including applications conventionally addressed bypiezoelectric materials, and for, e.g., watch springs, human-like musclefor robots, fuel injector actuators, eyeglass frames, head phones,shoes, e.g., as shoe inserts, fishing rods and fishing line shockleaders, sports equipment such as golf club shafts, helmets, andrackets, automotive radiators, air conditioners, e.g., as flapactuators, grill louvers for HVAC systems, rear view mirrors, toy andgreeting card motion actuators, solar concentrator actuators, utilityline snow and ice relief pulse actuators, drone control actuation,adaptive helicopter blade actuation, variable-geometry chevrons for jetengines, and mechanical actuation work, such as rock breaking;temperature sensors and temperature switches e.g., for cookingapplications and thermal applications such as nuclear plant safetysensing and actuation, window and window blind sensing and actuation,anti-scalding sensors, fire sensors and sprinkler actuation, andthermal-control for electrical current interrupters; desiccator sensorsand actuators, mechanical closures and latches, e.g., latch releases,e.g., for mechanical structures such as computers and tablets, orejector actuators, such as an ejector for a computer card or disc;mechanical control structures, such as for seat belt tighteners, gasmask deployment, camera focus and image stabilization, and touch-basedcommunication, such as for computer and phone haptics, e.g., invibration control and feedback, as in engine mount vibration control,earthquake damping in bridges and buildings, cell phone camera voicecoil damping springs, cell phone antenna, and active or passive drivetrain vibration control; micromotion as a micromotor, e.g., forapplications such as consumer disposable devices like toothbrushes andrazors; wearable electronics and clothing such as self-heating apparel,hat rims, and brassiere underwires; fasteners and couplers, such assatellite release bolts, pipe couplings, pumps and valves, e.g., inmicrofluidic applications such as microcircuit and LED cooling; pumpsand valves, e.g., for oil, gas, or water pumps, and blowout preventionvalves for oil and gas wells; and energy dissipation, e.g., in bodyarmor, in automotive frames, e.g., for bumper damping and crashabsorption, and damping as, e.g., damping felt, in heat engines, inthermoelastic cooling, and for other damping, e.g., acoustic damping.

FIG. 6 illustrates schematically an example SMA actuator 50 for whichthe surface conditioning methodology can be employed to produce adesired degree of energy damping. The example SMA actuator 50 isconfigured as an SMA ribbon, wire or similar structure 52 that isdisposed in a spring configuration in an actuation casing 54 having apiston 56 for producing a linear stroke. A bias spring 58 is provided inthe casing and can consist of any suitable metal or polymer, designedwith an appropriate stiffness. In a first condition (I), the SMA springstructure 52 is not activated, and is in the martensitic phase. In asecond condition (II), the SMA spring structure 52 has been activated byan activation stimulus, and is now in the austenitic phase. With thisphase transformation, the SMA spring produces a stroke of the piston 56.For this application, in which high fatigue life can be desired, andenergy dissipation is not in general required, the surface of the SMAwire is processed to be relatively smooth and thereby to not dissipatedamping energy to a large degree.

FIG. 7 schematically illustrates a contrasting example SMA deviceconfiguration 60 for which a relatively large degree of energy dampingcan be desired, here for vibration control. The configuration hereincludes a two-dimensional arrangement of SMA wires 62 that are woven orotherwise together arranged, e.g., as a fabric or other layer, and thatare connected between, e.g., structural supporting members 64, 66. If aforce 68 having at least one force component that is parallel to theplane of the woven arrangement is exerted against the structure 60,e.g., by an object such as an engine, ball, vehicle, person, bullet orother physical object, the wires 62 of the configuration 60 stretch andat least partially undergo a martensitic phase transformation,dissipating energy during the transformation. For this application, inwhich energy dissipation is preferred, the surface of the SMA wire isprocessed to be relatively rough, to thereby enhance damping ofmechanical energy in the configuration of woven wires.

These examples demonstrate that any suitable stimulus input can beemployed for causing the phase transformation in an SMA structure forwhich a conditioned surface is provided. In general, the phasetransformation can be initiated by application of stress to a given SMAstructure that is above the transformation temperature of the SMA. Thetemperature differential between the activated and non activated stateof the structure is preferably greater than about 2° C. In one example,the temperature range in which an oligocrystalline SMA structure cancyclically transform and be activated is between about −200° C. andabout +250° C.

But martensitic transformation can be induced by exposure of the SMAstructure to thermal, magnetic, electromagnetic, or other suitablestimulus environment. For example, a SMA structure can be actuatedthermally, by temperature control of a vapor or fluid atmospheresurrounding the structure. Alternatively, an SMA structuretransformation can be activated by passing electricity through thestructure to cause heating of the structure. Additionally, martensitictransformation can be triggered by the application of a magnetic fieldto the structure to produce a magnetic transformation hysteresis cycle.Both thermal and the magnetic transformation hysteresis damping cyclesare related to the energy dissipation of the cycles in the manner ofstress hysteresis being related to superelastic cycle transformation. Inall cases, the energy that is dissipated during a cycle transformationis controlled by surface conditioning of a SMA structure having afeature size that is in the surface-dominated size regime.

Example

Several samples of Cu—Zn—Al oligocrystalline wire with rough and smoothsurfaces were produced using the following method. Solid pieces of shapememory alloy with the composition Cu-22.9Zn-6.3Al (wt. %) were placed inan aluminosilicate glass tube that had a 4 mm inner diameter and aworking temperature of ˜1250° C. The inside of the tube was subjected tolow vacuum conditions and an oxy-acetylene burner was used to heat theglass/metal until the metal melted and the glass softened. The softenedglass capillary, with molten metal at its core, was then drawn out ofthe hot zone of the burner, reducing the capillary diameter andhardening the capillary. The result after this drawing process was aglass-coated metallic fiber having a diameter of 80 μm. The fiber wasannealed at 800° C. in an argon atmosphere for 3 h and water quenched.During the annealing the grains grew to span the wire cross section,forming a bamboo grain structure, and meeting the criterion foroligocrystallinity.

After annealing, the glass coating was removed by immersion in ˜10%diluted aqueous hydrofluoric acid. The surface of the wire after glassremoval was observed to be rather rough, with features reminiscent ofvalleys running parallel to the wire axis. The wires wereelectropolished in an electrolyte consisting of 67% phosphoric acid and33% deionized water, by volume, for 30-120 s depending on wire size. Theelectrolyte was stirred at 80 rpm using a magnetic stir bar to producecircular flow of the electrolyte. Two pure Cu electrodes were submergedin the electrolyte with the wire and connected in a circuit with a powersupply in which the voltage could be controlled and the currentmeasured. A polishing voltage of 2.8 V was set for the polishingprocess. One electrode, designated as the anode, was arranged with anend of the SMA wire attached thereto by conducting copper tape. The wirewas oriented along the flow lines of the electrolyte. The otherelectrode, designated as the cathode was provided with a larger surfacearea than the anode. With this polishing configuration, the power supplywas turned on for about 1 minute.

After electropolishing, the rough features of the as-drawn wire wereremoved; the surface was smooth and the wire diameter was uniform.Differential scanning calorimetry (DSC) of a polished wire with diameter65 μm determined that the martensitic transformation temperatures forthe wire were A_(f)˜25, A_(s)˜9, M_(s)˜8 and M_(f)˜6° C.

FIGS. 8A-8B are atomic force microscopy (AFM) topography images of thewire before and after polishing, respectively. Valleys running parallelto the wire axis characterize the unpolished wire. The polished wire,shown in FIG. 8B, exhibits a smooth surface in which the roughnessassociated with processing is removed. To obtain quantitative measuresof surface roughness there were determined the root mean square surfaceroughness, R_(q), and the arithmetic average surface roughness, R_(a),calculated after subtracting the wire curvature using a first orderflattening, based on the topography measurements in the conventionalmanner. The root mean square surface roughness parameter R_(q), wasfound to be 10 nm and 125 nm for the polished and unpolished wires,respectively. The arithmetic average roughness parameter, R_(a), wascalculated to be 7 nm and 88 nm for the polished and unpolished wires,respectively

To investigate the role of surface roughness on energy damping duringmartensitic SMA transformation, one of the as-drawn wires was cut intotwo halves and one half was electropolished as-above. The diameter ofthe unpolished wire (rough surface) was 80 μm and that of the polishedwire (smooth surface) was 41 μm, due to the removal of surface layers.Both of these wire diameters are in the surface-dominated feature sizeregime, as shown in the plot of FIG. 1A. The mechanical properties ofthe wire are therefore understood to be surface effect-dominated.

These two wires were then tested in tension at 35° C. in a dynamicmechanical analyzer, the DMA Q800 instrument from TA Instruments, NewCastle, Del., operated in load control at a loading rate of 10 MPa·min⁻¹during transformation. The gauge lengths were 8.2 and 5 mm for thepolished and unpolished wires, respectively.

FIG. 9A provides plots of the measured superelastic stress strain curvesof the rough and smooth wires for a first martensitic transformationcycle, i.e., the wire was not previously deformed. The slopes of thetransformation plateaus are similar, at about 600 MPa, but the forwardplateau is at a higher stress and the reverse plateau is at a lowerstress for the rough wire compared to the smooth wire. The stress toinduce martensite was about 26 MPa and 20 MPa for the rough and thesmooth wires, respectively, and the rough wire shows a much largerhysteresis size than the polished wire. The strain-averaged verticalhysteresis sizes were 21.5 MPa and 8.5 MPa for the rough and polishedwires, respectively; the energy dissipation of the two wires differ by afactor of 2.5. This demonstrates that the energy dissipated duringmartensitic transformation of the experimental wire could be controlled,here reduced by a factor of 2.5, by smoothing the surface of the wire.

The properties of Cu—Zn—Al and many other SMAs evolve over the course ofmultiple martensitic cycles before reaching a somewhat stable responseafter about ten cycles. FIG. 9B provides plots of the measuredsuperelastic transformation curves for the rough and smooth wires duringa tenth cycle of transformation, for which it is understood that thecurves reached a steady state. Interestingly, the forward plateaus arenow similar; however, the difference between the two reverse plateaus isstill large. In fact, the degree of energy dissipation still differs bya factor of 2.5, given that the hysteresis sizes are now 11.3 MPa and4.7 MPa for the rough and the polished samples, respectively.

The gauge sections of these two wire samples were estimated to be about40 grains and 65 grains, for the rough and polished wires, respectively,and it is therefore understood that factors such as grain size andorientation played only minor roles in affecting the dramaticdistinction in the measured SMA properties. Furthermore, given that bothsamples were cut from the same wire, the composition and internalmicrostructure, e.g. dislocation density, are assumed to be similar.Lastly, as demonstrated in the plotted regimes shown in FIG. 1A, givensimilar surface roughness, smaller wires in the surface-dominated sizeregime can exhibit larger hysteresis than larger wires, because suchwires have a higher surface-to-volume ratio and therefore more surfacearea. This size effect can be attributed to increased sampling ofobstacles at the wire surface by the austenite/martensite interface.Thus, it is especially suggestive that although the diameter of thepolished wire is finer, due to the removal of surface layers by theelectropolishing step, this sample still dissipates less energy per unitvolume than does the rough wire. After ruling out microstructural andcompositional differences as well as size effects, it is concluded thatthe difference in hysteresis between the two wires is attributable tothe difference in surface roughness.

With this description, it is demonstrated that for a SMA structure thatincludes a feature having a size in a surface-dominated size regimedefined for the SMA material of the structure, the amount of energydissipation during martensitic phase transformation can be controlled bycontrolling the roughness of the surface of the structure. The surfacecontrol methodology provides a practical way to optimize thetransformational performance of functional materials, withoutcompromising other properties, and is suitable for a wide range ofmaterials for obtaining performance results such as enhanced fatigueperformance.

It is recognized, of course, that those skilled in the art may makevarious modifications and additions to the embodiments described abovewithout departing from the spirit and scope of the present contributionto the art. Accordingly, it is to be understood that the protectionsought to be afforded hereby should be deemed to extend to the subjectmatter claims and all equivalents thereof fairly within the scope of theinvention.

We claim:
 1. A shape memory alloy wire comprising: a length of apolycrystalline shape memory alloy having an alloy composition includingat least one member selected from the group consisting of Cu in at leastabout 10 wt. %, Fe in at least about 5 wt. %, Au in at least about 5 wt.%, Ag in at least about 5 wt. %, Al in at least about 5 wt. %, In in atleast about wt. %, Mn in at least about 5 wt. %, Zn in at least about 5wt. % and Co in at least about 5 wt. %, the shape memory alloycomposition having a martensite crystal structure consisting of one of2H, 18R₁, M18R, and 6R; the length of a polycrystalline shape memoryalloy having a cross sectional wire diameter greater than about 1 micronand less than about 500 microns; the length of a polycrystalline shapememory alloy having an oligocrystalline morphology includingpolycrystalline shape memory alloy grains that span the cross sectionalwire diameter; and the length of a polycrystalline shape memory alloyhaving a wire surface with a surface roughness no greater than about 100nanometers.
 2. The shape memory alloy wire of claim 1 wherein the crosssectional wire diameter has an extent that causes energy dissipation bythe shape memory alloy wire during a martensitic phase transformation tobe dominated by surface roughness of the oligocrystalline shape memoryalloy wire.
 3. The shape memory alloy wire of claim 1 wherein the shapememory alloy wire cross sectional diameter is less than about 250microns.
 4. The shape memory alloy wire of claim 1 wherein the shapememory alloy wire cross sectional diameter is less than about 100microns.
 5. The shape memory alloy wire of claim 1 wherein the shapememory alloy wire cross sectional diameter is greater than about 10microns.
 6. The shape memory alloy wire of claim 1 wherein the shapememory alloy wire cross sectional diameter is greater than about 100microns.
 7. The shape memory alloy wire of claim 1 wherein the shapememory alloy composition comprises Cu—Zn—Al.
 8. The shape memory alloywire of claim 1 wherein the shape memory alloy composition comprisesCu-14Al-4Ni (wt. %).
 9. The shape memory alloy wire of claim 1 whereinthe shape memory alloy cross sectional wire diameter is greater thanabout 20 microns and less than about 250 microns.
 10. The shape memoryalloy wire of claim 1 wherein the length of polycrystalline shape memoryalloy has a surface roughness characterized by an arithmetic averagesurface roughness, R_(a), no greater than about 80 nanometers.